The 7E.17G9 monoclonal antibody reacts with mouse ICOS (Inducible COStimulatory molecule), a T cell specific molecule and a third member of the CD28/CTLA-4 family. A homodimer of 47-57 kDa, ICOS is expressed on activated T cells, has potent costimulatory activity for T cell activation and proliferation and is required for humoral immune response. ICOS binds to its ligand on activated APC including B cells called B7h/B7RP-1 and is thought to play a protective role in inflammatory autoimmune diseases. ICOS may be involved in the development of Th2 cells.